Light, lenses and the prism

A lens is a transparent body (glass, plastic) bounded on one or both sides by a spherical surface. A lens thicker in the middle than at the edge is convex; one thinner in the middle is concave. A convex lens makes parallel rays converge to a point (the focus), so it is a converging lens; a concave lens spreads rays out and is a diverging lens. Lenses are used in spectacles and contact lenses: a farsighted person wears convex lenses, a nearsighted person concave ones. A magnifier is a convex lens that gives a larger image when held close to an object; a microscope uses two lens systems (objective and eyepiece) to enlarge tiny objects, and total magnification is the product of the two. Cameras and telescopes also use lenses; the lens in our eye is a convex lens, the image forms upside down on the retina and the brain turns it the right way up. A water drop can act as a lens. Safety: a magnifier can focus sunlight to burn paper and harm eyes, so never point it at the Sun.